Process for producing a polyethylene coating on a substrate

ABSTRACT

This invention concerns a process for coating a substrate, and an extrusion coating structure. According to the present process, the coating is provided by producing a bimodal polyethylene composition by subjecting ethylene, optionally with hydrogen and/or comonomers to polymerization or copolymerization reactions in a multistage polymerization sequence of successive polymerization stages. The polymerization reactions are carried out in the presence of a single site catalyst capable of forming a composition which comprises a low molecular weight component with an MFR 2  of 20 g/10 min or more and a density higher than the density of the composition, and a high molecular weight component, said composition having a melt flow rate MFR 2  of 5 g/10 min or more and a density of 915–960 kg/m 3 . The composition is extruded on the substrate as such or by adding 10 wt-% or less, calculated from the total weight of the coating, of high pressure PE-LD by blending into the extrusion composition or by coextrusion.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI01/00161 which has an Internationalfiling date of Feb. 19, 2001, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a process for coating a substrate byextruding a polyethylene composition thereon. In addition, the presentinvention concerns an extrusion coating structure.

2. Description of Related Art

Low density polyethylene (PE-LD) made by a high pressure process(referred to as “high pressure PE-LD”) has been used conventionally forextrusion coating. High pressure PE-LD is easy to process, providesadequate moisture barrier and has good sealing properties. Themechanical properties of high pressure PE-LD are, however, not as goodas those of other PE grades. This means that a thicker layer of highpressure PE-LD is needed to achieve the mechanical properties requiredfor a coating.

Linear low density polyethylene (PE-LLD), medium density polyethylene(PE-MD) or high density polyethylene (PE-HD) materials exhibit bettermechanical properties. On the other hand, a PE composition with a higherdensity is more difficult to extrude, and the neck-in in extrusion istypically increased as the density increases.

The problem of the combination of good mechanical properties and goodprocessability has been tried to overcome by blending high pressurePE-LD with other PE grades. Another solution has been to coextrude HD,MD or LLD polyethylene together with PE-LD.

On the whole, it is desirable to try to decrease the total amount ofpolyethylene in a coated product due to the demands of environmentalregulations. In particular, it is of importance to decrease the amountof high pressure PE-LD in a coated product, because the production ofhigh pressure PE-LD in the world is decreasing. The present focus inpolyethylene production is in the low pressure processes, and thus mostoften the high pressure PE-LD has to be bought and brought to the plant.This increases the costs of the manufacturer of the extrusion coating.

WO 98/30628 discloses an extrusion coating structure comprising abimodal ethylene polymer. The ethylene polymer is a blend of at leasttwo different ethylene polymers and it contains 80–100% ethylenerepeating units and 20–0% alpha-olefin repeating units. The density ofthe ethylene polymer is 0.920–0.960 kg/m³.

The ethylene polymer blend can be made in a reactor sequence using asingle site or a Ziegler-Natta type catalyst. The catalyst used in theexamples is, however, not defined. It is to be noted that in theexamples the materials are either blended with 15% of high pressurePE-LD or coextruded with PE-LD. According to our experiments this wouldindicate that the material in the examples is produced usingZiegler-Natta catalysts. Nothing is mentioned of the advantages of usinga single site catalyst, especially of the possibility of extrusion ofthe composition without blending or coextruding with high pressure PE-LDwhen using a polyethylene composition produced by single site catalysts.

WO 96/16119 discloses a polyethylene extrusion composition comprisingfrom 75 wt-% to 95 wt-% of at least one ethylene/α-olefin interpolymercomposition having a density in the range of 850 kg/m³–940 kg/m³ andfrom 5 to 25 wt-% of high pressure ethylene polymer. The extrusioncomposition according to the publication has a melt index equal to orgreater than 1 g/110 min. It is stated in the publication that theethylene/α-olefin interpolymer can be prepared in any conventional way,inter al., by polymerizing in a reactor sequence using a homogeneoussingle site catalyst. The ethylene/α-olefin interpolymers disclosed inthe examples are substantially linear ethylene/1-octene copolymer and ahomogeneously branched linear ethylene/1-hexene copolymer. None of thesematerials is bimodal, and the melt flow properties obtained are due tothe long-chain branching of the materials. The catalyst most probablyused in the examples is called constrained geometry catalyst, but thisis not specifically stated. The polymerization conditions are notdefined, either.

U.S. Pat. No. 5,674,342 discloses a process for extrusion coating asubstrate. The composition used in the coating process consistsaccording to one alternative solely of ethylene polymer having a DowRheology Index (DRI) of at least 0.1. The ethylene polymer is, accordingto the examples a substantially linear polymer, which is not bimodal.According to the publication it is possible to use either single sitecatalysts or constrained geometry catalysts in the polymerization. It isstated in the publication that the constrained geometry catalysts arepreferably used and that preferably a solution polymerization process isemployed. In the examples, however, the used catalyst or processconditions are not indicated.

For the sake of completeness it should be mentioned that cast films(i.e. not extrusion coatings) have been made of substantially linearpolymers having a bimodal molecular weight distribution which have beenproduced by a metallocene catalyst in a two-stage solution process (WO99/09096). The polymers produced in the different stages were reportedto have equal densities. Specifically, the catalyst has the ability toproduce long chain branches in the polymer. The processibility of thepolymer on the film line was reported to be good. However, based on thedisclosure it is not possible to ascertain whether the prior polymerswould have a good processability on the extrusion coating line, wherethe line speed ins 100–500 m/min, compared to the film line having aline speed of a few meters per minute.

SUMMARY OF THE INVENTION

It is an aim of the present invention to eliminate the drawbacks ofprior art and to provide a novel process for coating a substrate byextrusion by producing such a bimodal extrusion composition ofpolyethylene, which can be extruded as such or with addition only ofsmall amounts of high pressure PE-LD either by blending or coextrusion.

The invention is based on the surprising finding that by polymerizingethylene in the presence of a single site catalyst in a multireactorsequence a polyethylene composition can be obtained which exhibitsimproved mechanical properties and can still be extruded on a substrateeither as such or by adding 10 wt-% or less of high pressure PE-LDeither by blending into the extrusion composition or by coextrusion.

More specifically, the present process is characterized by what isstated in the characterizing part of claim 1.

The extrusion coating structure is characterized by what is stated inthe characterizing part of claim 15.

A number of considerable advantages are obtained by means of the presentinvention. The present process enables the production of a compositionwhich can be used in extrusion coating either as such or blended orcoextruded with high pressure PE-LD, the amount of PE-LD being 10 wt-%or less of the total mass of the polyethylene coating. When less highpressure PE-LD is needed in the coating, the production of the coatingis cheaper.

The coating produced by the present process exhibits better mechanicalproperties than high pressure PE-LD and, thus, a thinner layer of thecoating is needed. This is in accordance with the public demand to useless polyethylene in the coating for environmental reasons. Inparticular, the tensile strength and tear strength in machine andtransverse directions are on a good level and superior to materialsintended for similar purposes. Also the optical properties are betterthan those of conventional materials; the gloss of the coating film ishigh and the haze is low.

The present extruded coating structure exhibits excellent resistance toenvironmental stress cracking. Surprisingly the resistance does notdecrease as a function of time. In packaging, this is a true benefitsince the manufacturers of liquid products need to be certain that theirproducts are packed so as to ensure a delivery of a non-contaminatedproduct to consumers.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

For the purpose of the present invention, “slurry reactor” designatesany reactor operating in slurry, in which reactor the polymer forms inparticulate form. As examples of suitable reactors can be mentioned acontinuous stirred tank reactor, a batch-wise operating stirred tankreactor or a loop reactor. According to a preferred embodiment theslurry reactor comprises a loop reactor.

By “gas phase reactor” is meant any mechanically mixed or fluidized bedreactor. Preferably the gas phase reactor comprises a mechanicallyagitated fluidized bed reactor with gas velocities of at least 0.2m/sec.

“Reaction zone” or “polymerisation zone” stands for one or severalreactors of similar type producing the same type or characteristics ofpolymer connected in the series.

By “melt flow rate” or abbreviated “MFR” is meant the weight of apolymer extruded through a standard cylindrical die at a standardtemperature in a laboratory rheometer carrying a standard piston andload. MFR is a measure of the melt viscosity of a polymer and hence alsoof its molecular weight. The abbreviation “MFR” is generally providedwith a numerical subindex indicating the load of the piston in the test.Thus, e.g., MFR₂ designates a 2.16 kg load. MFR can be determined using,e.g., by one of the following tests: ISO 1133 C4, ASTM D 1238 and DIN53735.

The Catalyst

The catalyst used in the polymerization process is a single sitecatalyst. According to a preferred embodiment, no fresh catalyst isadded to the second polymerization stage. The catalyst should produce abimodal, i.e., rather broad, molecular weight distribution and comonomerdistribution. Additionally, the catalyst should be able to produce amolecular weight of at least 150000–200000 g/mol in the high molecularweight polymer fraction, so that good mechanical properties areobtained. Some metallocene catalysts, like those based on a bis-(n-butylcyclopentadienyl)zirconium dichloride complex and disclosed inFI-A-934917 (=WO 95/12622) are not able to produce a high enoughmolecular weight polyethylene and their usefulness in bimodalpolymerization is limited. It has been found that some metallocenecatalysts are able to produce a high enough molecular weight. Oneexample of such catalysts is based on another complex disclosed in WO95/12622, having the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂)wherein

-   X₁ and X₂ are either same or different and are selected from a group    containing halogen, methyl, benzyl or hydrogen;-   Hf is hafnium;-   Cp is a cyclopentadienyl group; and-   R₁ and R₂ are the same or different and selected from the group of    linear and branched hydrocarbyl groups containing 1–10 carbon atoms.

Particularly suitable complexes of the kind described above arebis-(n-butyl cyclopentadienyl) hafnium dihalides. Another group ofsuitable complexes are the siloxy-substituted bridged bis-indenylzirconium dihalides, which are disclosed in FI-A-960437 (=WO 97/28170).

These catalysts are typically supported on a solid carrier, but they mayalso be used as unsupported. The carrier is typically inorganic, andsuitable materials comprise, e.g., silica (preferred), silica-alumina,alumina, magnesium oxide, titanium oxide, zirconium oxide and magnesiumsilicate (cf. also WO 95/12622). The catalysts are normally usedtogether with an aluminumoxane cocatalyst. Suitable cocatalysts are,e.g., methylaluminumoxane (MAO), tetraisobutylaluminumoxane (TIBAO) andhexaisobutylaluminumoxane (HIBAO). The cocatalyst is preferablysupported on the carrier, typically together with the catalyst complex,although the cocatalyst may optionally be fed into the reactorseparately.

A catalyst based on bis-(n-butyl cyclopentadienyl) hafnium dihalidecomplex supported on a silica or a silica-alumina carrier together witha methylaluminoxane cocatalyst is suitable to be run in a processincluding a loop rector and a gas phase reactor. Especially suitable isa catalyst based on bis-(n-butyl cyclopentadienyl) hafnium dichloride.Both the complex and the cocatalyst are supported on the carrier.

The thus obtained catalyst is then fed into the reactor. The catalyst iseasy to feed and the polymer settles well in the loop reactor. Thismakes the loop reactor operation easy.

In the gas phase reactor the catalyst is able to produce a sufficientlyhigh molecular weight material. This is essential to obtain the requiredprocessability on the film extrusion line and good mechanical propertiesof the film.

Polymerization Process

To produce the bimodal polyethylene composition used for extrusioncoating according to the invention, ethylene is polymerized in thepresence of a single site catalyst at elevated temperature and pressure.

Polymerization is carried out in a series of polymerization reactorsselected from the group of slurry and gas phase reactors. In thefollowing, the reactor system comprises one loop reactor (referred to as“the first reactor”) and one gas phase reactor (referred to as “thesecond reactor”), in that order.

However, it should be understood that the reactor system can comprisethe reactors in any number and order. It is also possible to conduct theprocess in two or more gas phase reactors.

The high molecular weight portion and the low or medium molecular weightportion of the product can be prepared in any order in the reactors. Itis to be understood that “high” and “low” are the components with a“higher” or “lower” molecular weight compared with each other. Aseparation stage is normally employed between the reactors to preventthe carryover of reactants from the first polymerization stage into thesecond one. The first stage is typically carried out using an inertreaction medium.

In every polymerization step it is possible to use also comonomersselected from the group of C₃₋₁₈ olefins, preferably C₄₋₁₀ olefins, suchas 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene,1-octene, 1-nonene and 1-decene as well as mixtures thereof and dienes,such as 1,5-hexadiene and 1,9-decadiene.

In addition to the actual polymerization reactors used for producing thebimodal ethylene homo- or copolymer, the polymerization reaction systemcan also include a number of additional reactors, such as prereactorsand postreactors. The prereactors include any reactor forprepolymerizing the catalyst and for modifying the olefinic feed, ifnecessary. The postreactors can be used, for example, for blendingadditional components such as the small amount of high pressure PE-LD tothe polymerization product. All reactors of the reactor system arepreferably arranged in series (in a cascade).

The polymerization steps may be performed in the most convenient order.Thus, it is possible to polymerize the low molecular weight component inthe first step of the process and the high molecular weight component inthe second step. It is also possible to perform the steps in a reversedorder, i.e., to polymerize the high molecular weight component in thefirst stage and the low molecular weight component in the second stage.If the first stage involves a slurry polymerization, it is preferred toproduce the low molecular weight component in that stage to avoidproblems due to the solubility of the polymer.

According to the present invention, the bimodal polyethylene compositionfor use in extrusion coating is preferably produced by

-   -   subjecting ethylene, optionally with hydrogen and/or comonomers,        to polymerization or copolymerization reactions in a multistage        polymerization sequence of successive polymerization stages,    -   carrying out the polymerization reactions in the presence of a        single site catalyst capable of forming a composition comprising    -   a low molecular weight component with an MFR₂ of 20 g/10 min or        more and a density higher than the density of the composition,        and    -   a high molecular weight component, said composition having a        melt flow rate MFR₂ of 5 g/ 10 min or more and a density of        915–960 kg/m³.

According to a preferred embodiment of the invention, the polymerizationcomprises the steps of

-   -   subjecting ethylene and optionally hydrogen and/or comonomers to        a first polymerization or copolymerization reaction in the        presence of a single-site catalyst in a first reaction zone or        reactor to produce a first polymerization product with an MFR₂        of 20 g/10 min or more;    -   recovering the first polymerization product from the first        reaction zone;    -   feeding the first polymerization product to a second reaction        zone or reactor;    -   feeding additional ethylene and, optionally, comonomers to the        second reaction zone;    -   subjecting the additional ethylene and optionally additional        monomer(s) and/or hydrogen to a second polymerization reaction        in the presence of the single-site catalyst and the first        polymerization product to produce a second polymerization        product having a MFR₂ of 5 g/10 min or more; and    -   recovering the combined polymerization product from the second        reaction zone.

In the first step of the process, ethylene with the optionalcomonomer(s) is fed into the first polymerization reactor. Along withthese components is optionally fed also hydrogen, which functions as amolecular weight regulator. The amount of hydrogen depends on thedesired molecular weight of the polymer. The catalyst may be fed to thereactor together with the reagents or, preferably, prior to thereagents, normally by flushing with a diluent.

The polymerization medium typically comprises the monomer (i.e.ethylene) and/or a hydrocarbon, in particular, a light inerthydrocarbon, such as propane, iso-butane, n-butane or isopentane. Thefluid is either liquid or gaseous. In the case of a slurry reactor, inparticular a loop reactor, the fluid is liquid and the suspension ofpolymer is circulated continuously through the slurry reactor, wherebymore suspension of polymer in particle form in a hydrocarbon medium ormonomer will be produced.

The conditions of the slurry reactor are selected so that 30–70 wt-%,preferably 40–60 wt-%, of the whole production is polymerized in theslurry reactor(s). The temperature is in the range of 40 to 110° C.,preferably in the range of 70 to 100° C. The reaction pressure is in therange of 25 to 100 bar, preferably 35 to 80 bar, and the mole fractionof ethylene in the liquid phase is typically 4–10% by mole. In order toproduce a polyethylene having a density in excess of 960 kg/m³, thepolymerization is preferably carried out at supercritical conditions attemperatures over 90° C. In slurry polymerization more than one reactorcan be used in series. In such a case the polymer suspension in areaction medium produced in the slurry reactor is fed without separationof inert components and monomers intermittently or continuously to thefollowing slurry reactor, which acts at lower pressure than the previousslurry reactor.

The polymerization heat is removed by cooling the reactor with a coolingjacket. The residence time in the slurry reactor must be at least 10minutes, preferably 20–100 min for obtaining a sufficient degree ofpolymerization.

As discussed above, if a low molecular weight polyethylene is thedesired product, hydrogen is fed into the reactor. With a catalystaccording to the present invention, a very small amount of hydrogen issufficient to produce a high MFR₂ polyethylene. Thus, an MFR₂ of 50–300g/110 min can be obtained with a hydrogen-to-ethylene feed ratio between0.1–0.5 kg of hydrogen/ton of ethylene. The hydrogen is typicallyconsumed in the reactor, so that the molar ratio of hydrogen to ethylenein the reaction mixture is between 0.4 and 1 kmol, or 400–1000 ppm byvolume.

After the first reaction zone, the volatile components of the reactionmedium are typically evaporated. As a result of the evaporation,hydrogen is removed from the product stream. The stream can be subjectedto a second polymerization in the presence of additional ethylene toproduce a high molecular weight polymer.

The second reactor is preferably a gas phase reactor, wherein ethyleneand preferably comonomers are polymerized in a gaseous reaction mediumin the presence of a single-site catalyst. If it is desirable to obtaina high molecular weight polymer, the polymerization is carried outessentially in the absence of hydrogen. The expression “essentially inthe absence of hydrogen” means, for the purposes of this invention, thatno additional hydrogen is fed to the reactor and the molar ratio ofhydrogen to ethylene in the reactor is lower than 0.6 mol/kmol (600 ppmby volume), preferably les than 0.4 mol/kmol (400 ppm by volume). Thegas phase reactor can be an ordinary fluidized bed reactor, althoughother types of gas phase reactors can be used. In a fluidized bedreactor, the bed consists of the formed and growing polymer particles aswell as still active catalyst come along with the polymer fraction. Thebed is kept in a fluidized state by introducing gaseous components, forinstance monomer and optionally comonomer(s) on such a flow rate thatwill make the particles act as a fluid. The fluidizing gas can containalso inert carrier gases, like nitrogen and propane and also hydrogen asa molecular weight modifier. The fluidized gas phase reactor can beequipped with a mechanical mixer.

The gas phase reactor used can be operated in the temperature range of50 to 115° C., preferably between 60 and 110° C. and the reactionpressure between 10 and 40 bar and the partial pressure of ethylenebetween 1 and 20 bar, preferably 5–10 bar.

The production split between the high molecular weight polymerizationreactor and the low molecular weight polymerization reactor is30–70:70–30. Preferably, 30 to 70 wt-%, in particular 40 to 60%, of theethylene homopolymer or copolymer is produced at conditions providing apolymer constituting the low molecular weight portion of the polymer,and 70 to 30 wt-%, in particular 60 to 40 wt-%, of the ethylenehomopolymer or preferably copolymer is produced at conditions providingthe high molecular weight portion of the polymer. The density of the lowmolecular weight portion is preferably into the polyethylene compositionprior to the extrusion. 930–975 kg/m³ and the density of the finalpolymer is preferably 915 to 960 kg/m³.

The present ethylene polymers and copolymers of ethylene can be blendedand optionally compounded with additives and adjuvants conventionallyused in the art. Thus, suitable additives include antistatic agents,flame retardants, light and heat stabilizers, pigments, processing aidsand carbon black. Fillers such as chalk, talc and mica can also be used.

The Extrusion Coating Composition

According to the present invention, the coating of a substrate iscarried out by extruding the bimodal polyethylene composition obtainedby the polymerization process described above on the substrate.

The polyethylene composition comprises, according to the invention, 30to 70 wt-%, preferably 40 to 60 wt-%, and in particular 45 to 55 wt-% ofa high molecular weight portion, and 70 to 30 wt-%, preferably 60 to 40wt-%, and in particular 55 to 45 wt-% of a low molecular weight portion.The melt flow rate MFR₂ of the composition is typically 5 g/10 min ormore, preferably in the range of 5–50 g/10 min, and in particular in therange of 10–20 g/10 min.

The density of the polymer product is from about 915 kg/m³ to about 960kg/m³.

The density of the polymer and the density and the melt flow rate of thelow molecular weight component correlate preferably as presented in thefollowing:

If the density of the composition is between 940–960 kg/m³, the MFR₂ ofthe low molecular weight component is preferably between 70–1000 g/10min and the density is higher than 965 kg/m³.

If the density of the composition is in the medium density area, i.e.,between 930–940 kg/m³, the low molecular weight component preferably hasan MFR₂ between 50–1000 g/10 min and a density between 940–975 kg/m³.

If the density of the composition is low, between 915–930 kg/m³, the lowmolecular weight component preferably has an MFR₂ between 20–500 g/10min and density between 935–965 kg/m³.

The Extrusion

The polymer obtained from the polymerization process is fed, typicallyin the form of powder or pellets, optionally together with additives, toa extruding device. The extruding device is known as such and anyextruding device known in the art may be used.

As already discussed, according to the present invention the film can beextruded without adding high pressure PE-LD. Thus, by means of thepresent invention it is possible to provide a single layer coatingconsisting essentially of bimodal polyethylene on a substrate. By“single layer coating” is meant that the coating is provided withoutcoextrusion with some other polymer. Further, “consisting essentially ofbimodal polyethylene” means that no high pressure PE-LD is added to theextrusion coating composition and that the composition does not containsignificant amounts of other polymers, either.

According to a first embodiment of the present invention high pressurePE-LD is added to the polymerization product obtained from thepolymerization process. The polymerization product and high pressurePE-LD can be blended, e.g., in a postreactor of the polymerizationprocess (cf. above) or in the extruder.

According to a second embodiment the polymerization product obtainedfrom the process is coextruded with high pressure PE-LD. The coextrusionis known in the art as such and any extruding device suitable forcoextrusion may be used.

By adding high pressure PE-LD to the composition it is possible toenhance the processability of the extrusion composition. It is to benoted, however, that the polymer composition obtained from thepolymerization process described above already has a good processabilityin extrusion. Therefore, if it is decided to use high pressure PE-LD,the amount can, and should, be kept low. The amount of high pressurePE-LD, calculated as the weight percentage of the high pressure PE-LD inthe polyethylene coating, is typically 10 wt-% or less, preferably 0.1–8wt-%, in particular 0.1–5 wt-% and most preferably 0.1–3 wt-%.

The coating is typically extruded on a fiber based substrate material,such as paper or paperboard (cardboard). The substrate can also be afilm made of polyester, cellophane, polyamide, polypropylene or orientedpolypropylene. The thickness of the film is typically 10–80 μm. Further,the substrate can be an aluminium foil, typically of a thickness from 6μm to 300 μm.

The thickness of the coating is typically about 10–200 μm, preferably30–100 μm. The coating weight is typically in the range of 10–100 g/m²,preferably in the range of 20–50 g/m². The composition according to theinvention can also be run into a film on a cast film line. A 40 μm castfilm produced in this way generally exhibits the following features:

-   -   haze 10% or less, preferably 8% or less, and in particular 5% or        less;    -   gloss 70% or more, preferably 100% or more, and in particular        120% or more;    -   puncture strength of 900 N/mm or more, preferably 1000 N/mm or        more and, in particular, 1200 N/mm or more;    -   tensile strength machine/transverse direction 20/20 MPa or more,        preferably 22/21 MPa or more; and    -   tear strength machine/transverse direction 1.0/1.5 MPa or more,        preferably 1.6/1.8 MPa or more, and in particular 1.6/2.0 MPa or        more.

The environmental stress cracking resistance, ESCR, values of the filmsextruded from the present composition are typically in the range of200–1000%, preferably in the range of 400–800% and in particular in therange of 550–700%. What is to be specifically noted is that thepercentage increases as the time for exposure increases at least up to 4weeks.

The odour and taste properties, which necessarily are determined afterthe coating process, are in the same level as those of known materials.

Description of Analytical Methods

All the properties under “End product properties” in Table 1 and allfilm properties in Table 2 are determined from the extruded film. Thefilm is obtained by first coating a substrate and then tearing thecoating from the substrate.

Odour

Odour is determined from a structure consisting of the test materialcoated on a polyester film. The coating weight is 40 g/m², coating speedis 100 m/min and the coating is conducted at 315° C. temperature. Afterthe run the coating is removed from the polyester film and cut to A4size samples. The samples are put to flasks and aged in an oven at 40°C. for 0.5 hours and 1 hour. The test is carried out by two groups ofpeople, each group consisting of four persons. The persons smell thesamples and evaluate their odour in scale 0–3 (0=odourless).

Taste

Taste is determined from a structure consisting of the test materialcoated on an aluminium film. The coating conditions are similar to thosein the odour test. The structure is cut to three 240 mm times 320 mmpieces, which are sealed into bags. Into the bag is poured 150 ml ofwater and the samples are put into an oven at 40° C. for two hours. Thetesting is done by two groups, as in the odour test, who taste the waterand rank it in scale 0–3 (0=tasteless).

Environmental Stress Cracking Resistance, ESCR

The environmental stress cracking resistance is measured as theelongation % of a 40 μm film after contact with 10% Igepal liquid at 50°C. for the indicated time period.

Neck-In

The difference between the width of the die and the final coating width.

Gloss

Gloss is measured according to ASTM D 2457v.

Haze

Haze is measured according to ASTM 1003.

Secant Modulus

Secant modulus at 1% elongation is obtained from a tensile experiment,performed according to ISO 1184. The specimen is extended along itsmajor axis at a constant speed. A stress-strain curve is thus obtained.The value of the secant modulus is the ratio of stress to strain at 1%strain on the stress-strain curve.

Strain at Break

Strain at break is also obtained from a tensile experiment. This is thevalue of the strain at the point where the specimen breaks.

Hot Tack

Hot tack is measured by measuring the force which is needed to tearapart a heat sealed structure before the material has been allowed tocool. Sealing is made at different temperatures (temperature isincreased by 5 or 10° C. intervals) and the sealing pressure is 1.0N/mm². Sealing is made in 0.2 seconds. Tearing is started after 0.1seconds from sealing and the tearing rate is 200 mm/s.

Puncture

The puncture test has been carried out according to ASTM D4649.

Tensile Strength

The experiment is performed according to ISO 1184 method. The specimenis extended along its major axis at a constant speed. Normal 50 mm couldbe used as a distance between grips (gauge length) in film tensiletesting. 125 mm gauge length would be required for tensile modulusmeasurement so this was not possible for 100 mm cast film in transversedirection.

Tear Strength

Tear testing is done according to ASTM 1922. Machine direction iseasier, as the thickness variation in critical test direction is bettercontrolled. Thickness varied more in transverse direction andoccasionally difficulties arise in taking the sample in a manner whichguarantees an even thickness for the critical testing area.

WVTR

The water vapour transmission rate is measured in a diffusion cell whereabove the sample flows a stream of dry air and below the sample is anatmosphere with a constant humidity at a constant temperature (usually37.8° C.). When the sample has reached an equilibrium, the air stream isdirected to an IR detector determining the water content of the airstream. In this way can the water vapour transmission rate be measured.This method gives approximately five times higher WVTR values than ASTME-96.

The invention is further illustrated with the aid of the followingexamples.

EXAMPLE 1 Catalyst Preparation

168 g of a metallocene complex (bridged siloxy-substituted bis-indenylzirconium dichloride, according to WO 97/28170) and 9.67 kg of a 30% MAOsolution supplied by Albemarle were combined and 3.18 kg dry, purifiedtoluene was added. The thus obtained complex solution was added on 9 kgsilica carrier SP9–243 by Grace having a particle size of 20 microns,pore volume of 1.5–1.7 mm³ and specific surface area of 350–400 mm²/g.The complex was fed very slowly with uniform spraying during 2 hours.Temperature was kept below 30° C.

The thus obtained catalyst was dried under nitrogen for 6 h at 75° C.temperature. After nitrogen drying, the catalyst was further dried undervacuum for 10 h.

EXAMPLE 2

Into a continuously operating 500 dm³ loop reactor, operated at 85° C.temperature and 60 bar pressure, were introduced propane diluent,ethylene, 1-butene comonomer and hydrogen together with a single sitepolymerization catalyst described in Example 1 so that 25 kg/h ofethylene copolymer having an MFR₂ of 90 g/10 min and a density of 936kg/m³ was produced. The polymer slurry was withdrawn from the reactor,the hydrocarbons were flashed off and the polymer was introduced intothe gas phase reactor operated at 75° C. temperature and 20 barpressure. Additional ethylene and 1-butene comonomer were introducedinto the reactor so that the ethylene copolymer having an MFR₂ of 5.6g/10 min and a density of 924 kg/m³ was withdrawn from the reactor at arate of 51 kg/h. The polymer was compounded in a corotating intermeshingtwin screw extruder and pelletized.

The pelletized product was extruded on a substrate. The properties ofthe extrusion composition and the extruded film are presented in Table1.

EXAMPLE 3 (COMPARATIVE)

A polyethylene composition of commercial high pressure PE-LD wasextruded on a substrate. The properties of the composition and theextruded film are presented in Table 1.

EXAMPLE 4 (COMPARATIVE)

Into a continuously operating 500 dm³ loop reactor, operated at 85° C.temperature and 60 bar pressure, were introduced propane diluent,ethylene, 1-butene comonomer and hydrogen together with a Ziegler-Nattapolymerization catalyst prepared according to Example 3 of WO 95/35323,so that 25 kg/h of ethylene copolymer having an MFR₂ of 100 g/10 min anda density of 941 kg/m³ was produced. The polymer slurry was withdrawnfrom the reactor, the hydrocarbons were flashed off and the polymer wasintroduced into the gas phase reactor operated at 75° C. temperature and20 bar pressure. Additional ethylene and 1-butene comonomer wereintroduced into the reactor so that ethylene copolymer having an MFR₂ of10 g/l 0 min and a density of 931 kg/m³ was withdrawn from the reactorat a rate of 51 kg/h. The polymer was compounded in a corotatingintermeshing twin screw extruder and pelletized.

The pelletized product was extruded on a substrate. The properties ofthe extrusion composition and the extruded film are presented in Table1.

TABLE 1 Ex.2 Ex.3 Ex.4 MFR₂ g/10 min 5.6 7.5 12 Dens. kg/m³ 924 920 932Film appearance — +++ — Melt T ° C. 315 315 285 Max rpm 117 >200 164Coat. weight variation g/m² 9–11 0 8–10 (coex 5 + 5 g/m², 200 m/min) N-Imm 135 65 114 End product properties Odor (1–3) 1.0 1.3 0.9 BTM 14137Taste (1–3) 1.3 1.4 0.7 BTM 14138 ESCR, % 0 week 580 160 650 2 weeks 60010 750 4 weeks 670 4 470 Hot tack N/° C.  2.9/115  2.5/105  4.2/120Mechanicals, 40 μm cast Tens 1% secmod md/cd Mpa 148/158 130/135 227/236Strain a break % 740/710 500/620 920/970 Tens. strength md/cd MPa  23/21.5 18/17 21/20 Tear md/cd N 1.68/2.48 1.60/1.71 0.94/0.94 Punct.N/mm 1215 1105 853 Haze % 3.4 6.8 16.7 Gloss 126 83 105 WVTR* g/m²* 24 h13.9 13.5** 13.3 *Co-ex 10 gsm high pressure PE-LD of Ex. 3 + 20 gsmtest material; RH 90%; T 38° C. **LE4524 30 gsm mono

The processability of the coating composition according to the presentinvention is on the same level as for the bimodal polyethylene producedin the presence of a Ziegler-Natta catalyst. The processability of highpressure PE-LD is, as expected, slightly better.

ESCR of both bimodal materials is superior to that of high pressurePE-LD. The composition according to the present invention has a superiortime behaviour compared with the bimodal Ziegler-Natta material.

Secant modulus is a function of the density of the material, which isalso verified by the values shown in Table 1. The same seems to applyfor strain at break.

The material according to the present invention exhibits higher tensileand tear strengths both in machine and transverse directions than eitherof the two other materials. Also the puncture strength of the materialproduced according to the present invention is higher than those of thereference materials.

The optical properties, haze and gloss, are better than those of theother two materials.

The differences in initial sealing temperatures (hot tack) are probablydue to differences in density.

EXAMPLE 5 Coextrusion Tests

The sample ‘Single site catalyst’ is a sample produced by polymerizingethylene and 1-butene comonomer in two cascaded gas phase reactors inthe presence of a catalyst prepared according to WO 95/12622, comprisinga bis-(n-butylcyclopentadienyl)zirconium dichloride complex and MAO on asilica carrier. After the first reactor the MFR₂ of the polymer was 100g/10 min and the density was 945 kg/m³. The final polymer had an MFR₂ of6 g/10 min and a density of 935 kg/m³. The production split was 50/50.

The sample has been run on a Beloit line in coextrusion, with 15 g/m²PE-LD and 15 g/m² the sample of the material according to the invention.Thus, the amount of PE-LD is not in accordance with the presentinvention, but the aim of this example is to compare the properties of amaterial produced with a single site catalyst and a material producedwith a Ziegler-Natta catalyst. A reference run was made using 30 g/m²PE-LD only. The physical properties of the coated paper were determinedfrom both runs.

The sample ‘Ziegler-Natta’ is a sample produced using a Ziegler-Nattacatalyst according to Example 3 of WO 98/30628. The sample has MFR₂=9and density=931 kg/m³.

The sample has been run on a Beloit line in coextrusion, with 5 g/m²PE-LD and 15 g/m² of the Ziegler-Natta material. A reference run wasmade using 20 g/m² PE-LD only. The physical properties of the coatedpaper were determined from both runs.

Since the coating weights of the SSC and Z-N materials have beendifferent, a direct comparison of the values of the mechanicalproperties would not be feasible. Instead, the figures of both materialswere compared to a reference run made with the same coating weight usingPE-LD. The values of tensile strength, elongation, tear strength andburst force indicate what is the value of the property of the linearcoextruded coating compared to the PE-LD reference. Thus, a figure 190%indicates, that the property has been improved by 90% compared to PE-LD.

Table 2 indicates, that with respect to tensile strength and elongation,both materials give a similar improvement compared to the PE-LD. Withrespect to tear strength and burst force, the SSC material seems to givea bigger improvement.

The hot tack and cold tack values are similar, measured from similarsamples. The results indicate that the Ziegler-Natta material gives aslightly higher force, but the single site material gives a lowertemperature. The latter result is even more positive when consideringthe higher density of the single site catalyst material sample (935kg/m³ vs. 931 kg/m³).

TABLE 2 Ziegler-Natta Single site catalyst MFR₂ g/10 min  9  6 Densitykg/m³ 931 935 Tensile strength MD  83% 109% TD 143% 149% Elongation MD190% 204% TD 164% 136% Tear strength MD 107% 145% TD 109% 162% Burstforce 176% 210% Hot tack Force, N  3.9  3.2 Temp., ° C. 120 115 Coldtack Force, N  8.5  6.2 Temp., ° C. 125 115 WVTR g/m²/24 h  13.5

1. A process for coating a substrate, comprising producing a bimodalpolyethylene composition by subjecting ethylene, optionally withhydrogen and/or comonomers to polymerization or copolymerizationreactions in a multistage polymerization sequence of successivepolymerization stages; carrying out the polymerization reactions in thepresence of a single site catalyst capable of forming a compositioncomprising a low molecular weight component with an MFR₂ of 20 g/10 minor more and a density higher than the density of the composition, and ahigh molecular weight component, said composition having a melt flowrate MFR₂ of 10–20g/10 min and a density of 915–960 kg/m³; and extrudingsaid composition on the substrate as such or by adding 10 wt % or less,calculated from the total weight of the coating, of high pressure PE-LDby blending into the extrusion composition or by co-extrusion whereinthe active complex of said single site catalyst has the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂), where X₁ and X₂ are thee same or different andselected from the group comprising halogen, methyl, benzyl, andhydrogen; Hf is hafnium; Cp is cyclopentadienyl group; and R₁ and R₂ aresame or different and stand for linear and branched hydrocarbyl groupscontaining 1–10 carbon atoms.
 2. The process according to claim 1,wherein the bimodal polyethylene composition is produced by subjectingethylene, optionally with hydrogen and/or comonomers to a firstpolymerization or copolymerization reaction in the presence of asingle-site catalyst in a first reaction zone or reactor to produce afirst polymerization product with an MFR₂ of 20 g/10 min or more;recovering the first polymerization product from the first reactionzone; feeding the first polymerization product to a second reaction zoneor reactor; feeding additional ethylene and, optionally, comonomers tothe second reaction zone; subjecting the additional ethylene andoptionally additional monomer(s) and/or hydrogen to a secondpolymerization reaction in the presence of the single-site catalyst andthe first polymerization product to produce a second polymerizationproduct having a MFR₂ of 10–20 g/10 min; and recovering the combinedpolymerization product from the second reaction zone.
 3. The processaccording to claim 1, wherein the substrate is paper or paperboard. 4.The process according to claim 1, wherein the active complex of thecatalyst is bis-(n-butyl cyclopentadienyl) hafnium dihalide.
 5. Theprocess according to claim 1, wherein essentially no fresh catalyst isadded to the reactors other than the first one.
 6. The process accordingto claim 1, wherein the process is carried out in a polymerizationreactor cascade comprising a loop reactor and a gas phase reactor, inthat order.
 7. The process according to claim 1, wherein the process iscarried out in a polymerization reactor cascade comprising two or moregas phase reactors.
 8. The process according to claim 1 or 2, wherein0.1–5 wt-%, calculated from the total weight of the coating compositionof high pressure PE-LD is blended into polyethylene composition prior tothe extrusion.
 9. The process according to claim 1 or 2, wherein 0.1–3wt-%, calculated from the total weight of the coating composition ofhigh pressure PE-LD is blended into the polyethylene composition priorto the extrusion.
 10. A process for coating a substrate, comprising:producing a bimodal polyethylene composition by subjecting ethylene,optionally with hydrogen and/or comonomers to polymerization orcopolymerization reactions in a multistage polymerization sequence ofsuccessive polymerization stages; carrying out the polymerizationreactions in the presence of a single site catalyst capable of forming acomposition comprising a low molecular weight component with an MFR₂ of20 g/10 min or more and a density higher than the density of thecomposition, and a high molecular weight component, said compositionhaving a melt flow rate MFR₂ of 10–20 g/10 min and a density of 915–960kg/m³; extruding said composition on the substrate as such or by adding10 wt % or less calculated from the total weight of the coating, of highpressure PE-LD by blending into the extrusion composition or byco-extrusion, wherein the active complex of the catalyst is asiloxy-substituted bis-indenyl zirconium dihalide.
 11. The processaccording to claim 10, wherein a metallocene catalyst is used as thesingle site catalyst.
 12. A process for coating a substrate, comprising:producing a bimodal polyethylene composition by subjecting ethylene,optionally with hydrogen and/or comonomers to polymerization orcopolymerization reactions in a multistage polymerization sequence ofsuccessive polymerization stages; carrying out the polymerizationreactions in the presence of a single site catalyst capable of forming acomposition comprising a low molecular weight component with an MFR₂ of20 g/10 min or more and a density higher than the density of thecomposition, and a high molecular weight component, said compositionhaving a melt flow rate MFR₂ of 10–20 g/10 min and a density of 915–960kg/m³; and extruding said composition on the substrate as such or byadding 10 wt % or less, calculated from the total weight of the coating,of high pressure PE-LD by blending into the extrusion composition or byco-extrusion, wherein 0.1–8 wt %, calculated from the total weight ofthe coating composition, of high pressure PE-LD is blended into thepolyethylene composition prior to the extrusion, wherein 30 to 70% ofthe ethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of 10 g/10 min or more and 70% of theethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of less than 10 g/10 min or more and 70to 30% of the ethylene homopolymer or copolymer is produced atconditions which provide a polymer having a MFR₂ of less than 5 g/10min.
 13. The process according to claim 10, wherein 40 to 60% of theethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of 10 g/10 min or more and 60 to 40% ofthe ethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of less than 5 g/10 min.
 14. The processaccording to claim 1 or 2, wherein the composition is coextruded on thesubstrate together with high pressure PE-LD so as to form a coatingcomprising 0.1–8 wt-%, calculated from the total weight of the coating,of high pressure PE-LD.
 15. The process according to claim 14, whereinsaid coating composition comprises 0.1–5 wt-%, calculated from the totalweight of the coating of high pressure PE-LD.
 16. The process accordingto claim 14, wherein said coating composition comprises 0.1–3 wt-%,calculated from the total weight of the coating of high pressure PE-LD.17. A process for coating a substrate, comprising producing a bimodalpolyethylene composition by subjecting ethylene, optionally withhydrogen and/or comonomers to polymerization or copolymerizationreactions in a multistage polymerization sequence of successivepolymerization stages; carrying out the polymerization reactions in thepresence of a single site catalyst capable of forming a compositioncomprising a low molecular weight component with an MFR₂ of 20 g/10 minor more and a density higher than the density of the composition, and ahigh molecular weight component, said composition having a melt flowrate MFR₂ of 10–20 g/10 min and a density of 915–960 kg/m³; extrudingsaid composition on the substrate as such or by adding 10 wt % or less,calculated from the total weight of the coating, of high pressure PE-LDby blending into the extrusion composition or by co-extrusion, whereinthe active complex of the catalyst is a siloxy-substituted bis-indenylzirconium dihalide.
 18. A process for coating a substrate, comprisingproducing a bimodal polyethylene composition by subjecting ethylene,optionally with hydrogen and/or comonomers to polymerization orcopolymerization reactions in a multistage polymerization sequence ofsuccessive polymerization stages; carrying out the polymerizationreactions in the presence of a single site catalyst capable of forming acomposition comprising a low molecular weight component with an MFR₂ of20 g/10 min or more and a density higher than the density of thecomposition, and a high molecular weight component, said compositionhaving a melt flow rate MFR₂ of 10–20 g/10 min and a density of 915–960kg/m³; extruding said composition on the substrate as such or by adding10 wt % or less, calculated from the total weight of the coating, ofhigh pressure PE-LD by blending into the extrusion composition or byco-extrusion, wherein 0.1–0.8 wt %, calculated from the total weight ofthe coating composition, of high pressure PE-LD is blended into thepolyethylene composition prior to the extrusion, wherein 30–70% of theethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of 10 g/10 min or more and 70–30% of theethylene homopolymer or copolymer is produced at conditions whichprovide a polymer having a MFR₂ of less than 5 g/10 min; wherein theactive complex of said single site catalyst has the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂), where X₁ and X₂ are thee same or different andselected from the group comprising halogen, methyl, benzyl, andhydrogen; Hf is hafnium; Cp is cyclopentadienyl group; and R₁ and R₂ aresame or different and stand for linear and branched hydrocarbyl groupscontaining 1–10 carbon atoms. R₁ and R₂ are same or different and standfor linear and branched hydrocarbyl groups containing 1–10 carbon atoms.19. The process according to claim 18, wherein the substrate is paper orpaperboard.
 20. A process for coating a substrate, comprising producinga bimodal polyethylene composition by subjecting ethylene, optionallywith hydrogen and/or comonomers to polymerization or copolymerizationreactions in a multistage polymerization sequence of successivepolymerization stages; carrying out the polymerization reactions in thepresence of a single cite catalyst capable of forming a compositioncomprising a low molecular weight component with a MFR₂ of 20 g/10 minor more and a density higher than 965 kg/m³, but higher than the densityof the composition, and a high molecular weight component, saidcomposition having a melt flow rate MFR₂ of 10–20 g/10 min and a densityof 940–960 kg/m³; and extruding said composition on the substrate assuch or by adding 10 wt-% or less, calculated from the total weight ofthe coating, of high pressure PE-LD by blending into the extrusioncomposition or by co-extrusion wherein the active complex of said singlesite catalyst has the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂), where X₁ and X₂ are thee same or different andselected from the group comprising halogen, methyl, benzyl, andhydrogen; Hf is hafnium; Cp is cyclopentadienyl group; and R₁ and R₂ aresame or different and stand for linear and branched hydrocarbyl groupscontaining 1–10 carbon atoms.
 21. The process according to claim 20,wherein the bimodal polyethylene composition is produced by subjectingethylene, optionally with hydrogen and/or comonomers to a firstpolymerization or copolymerization reaction in the presence of asingle-site catalyst in a first reaction zone or reactor to produce afirst polymerization product with an MFR₂ of 20 g/10 min or more;recovering the first polymerization product from the first reactionzone; feeding the first polymerization product to a second reaction zoneor reactor; feeding additional ethylene and, optionally, comonomers tothe second reaction zone; subjecting the additional ethylene andoptionally additional monomer(s) and/or hydrogen to a secondpolymerization reaction in the presence of the single-site catalyst andthe first polymerization product to produce a second polymerizationproduct having a MFR₂ of 5 g/10 min or more; and recovering the combinedpolymerization product from the second reaction zone.
 22. The processaccording to claim 20 or 21, wherein said composition has a density of915–930 kg/m³, and the low molecular weight component has a MFR₂ of20–500 g/10 min and a density of 935–965 kg/m³.
 23. A process forcoating a substrate, comprising: producing a bimodal polyethylenecomposition by subjecting ethylene, optionally with hydrogen and/orcomonomers to polymerization or copolymerization reactions in amultistage polymerization sequence of successive polymerization stages;carrying out the polymerization reactions in the presence of a singlecite catalyst capable of forming a composition comprising a lowmolecular weight component with an MFR₂ of 70–1000 g/10 min or more anda density higher than 965 kg/m³, and a high molecular weight component,said composition having a melt flow rate MFR₂ of 10–20 g/10 min and adensity of 940–960 kg/m³; and extruding said composition on thesubstrate as such or by adding 10 wt-% or less, calculated from thetotal weight of the coating, of high pressure PE-LD by blending into theextrusion composition or by co-extrusion; wherein the active complex ofsaid single site catalyst has the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂), where X₁ and X₂ are thee same or different andselected from the group comprising halogen, methyl, benzyl, andhydrogen; Hf is hafnium; Cp is cyclopentadienyl group; and R₁ and R₂ aresame or different and stand for linear and branched hydrocarbyl groupscontaining 1–10 carbon atoms.
 24. A process for coating a substrate,comprising producing a bimodal polyethylene composition by subjectingethylene, optionally with hydrogen and/or comonomers to polymerizationor copolymerization reactions in a multistage polymerization sequence ofsuccessive polymerization stages; carrying out the polymerizationreactions in the presence of a single site catalyst capable of forming acomposition comprising a low molecular weight component with an MFR₂ of50–1000 g/10 min or more and a density in the range of 940 to 975 kg/m³but higher than the density of the composition, and a high molecularweight component, said composition having a melt flow rate MFR₂ of 10–20g/10 min and a density of 930–940 kg/m³, and extruding said compositionon the substrate as such or by adding 10 wt-% or less, calculated fromthe total weight of the coating, of high pressure PE-LD by blending intothe extrusion composition or by co-extrusion; wherein the active complexof said single site catalyst has the general formula(X₁)(X₂)Hf(Cp-R₁)(Cp-R₂), where X₁ and X₂ are thee same or different andselected from the group comprising halogen, methyl, benzyl, andhydrogen; Hf is hafnium; Cp is cyclopentadienyl group; and R₁ and R₂ aresame or different and stand for linear and branched hydrocarbyl groupscontaining 1–10 carbon atoms.